This invention relates to a capacitance type switch and more particularly, to a capacitance type switch having a dust proof structure.
A heretofore known capacitance type switch includes flat sheet-like fixed electrodes and a movable electrode coated with a thin insulating film, and is turned to its on state when the movable electrode is brought close to the fixed electrodes so that these fixed electrodes are coupled by electrostatic capacitance via the movable electrode. The switch is turned to its off state when the movable electrode is withdrawn far enough from the fixed electrodes so as to substantially eliminate the electrostatic capacitance between the two fixed electrodes. To maximize the electrostatic capacitance generated when the movable electrode is pushed towards the fixed electrodes and to keep the capacitance type switch under a stable ON state, both movable electrode and fixed electrodes have a flat sheet-like form so that they come into intimate contact with one another via the insulating film. If contact between them is somehow deteriorated as when dust in the air or the like builds up on the movable electrode or on the fixed electrodes, the performance of the capacitance switch may deteriorate to a point where the switch will not function. For this reason, a capacitance type switch generally has a dust proof structure.
FIG. 1 is a sectional view of a typical example of the heretofore known capacitance switches. A plurality of capacitance switches are generally disposed on a printed circuit board, such as the bottom plate of a keyboard, represented by reference numeral 1, though only one switch is shown in the drawing. Two fixed electrodes 2, 3 are disposed on the printed circuit board 1 in a spaced-apart relation so that one serves as the input with the other serving as the output. A case 4 of synthetic resin is fitted to the printed circuit board 1 to encase the fixed electrodes 2, 3, and is equipped with a window 4a at its upper end. A stem 5 of synthetic resin penetrates through this window 4a. A flange 5a is formed at the lower end of this stem 5 and a hole 5b is formed at the center of the lower end of the stem. A key top 6 having a display character or mark on the surface is fitted to the upper end of the stem 5. The upper end of a rubber cover 7 fits into the hole 5b at the lower end of the stem 5. This cover 7 has a bell-like shape as depicted in FIG. 2 and has a notch 7a for air to flow through at its lower end. A movable electrode member 8 made of metal is fitted inside the cover 7, with an insulating layer 9 consisting of a synthetic resin film formed on the lower surface of the electrode member 8.
In the state shown in FIG. 1, the stem 5 and the electrode member 8 are urged upwardly by the resiliency of the cover 7' and the switch is kept in its off state. To turn the switch to its on state the key top 6 is depressed to push the stem 5 downwards. At this time, the cover 7 is depressed and the air inside it jets out through the notch 7a. When the key top 6 is released from the pushing force, the cover 7 pushes the stem 5 upwardly by its resiliency and returns to the state shown in FIG. 1. At this time, any fine dust that may have entered the case from the gap between the stem 5 and the window 4a of the case and has stayed there is sucked into the cover through the notch 7a and reaches the fixed electrodes 2, 3 and the electrode member 8, thus impeding smooth switch operation from time to time.
To eliminate such a problem, one possible counter-measure would be to interpose the cover 7 between the lower end of the key top 6 and the upper surface of the case 4 so as to encase the stem 5 and the window 4a. However, because the notch 7a communicates the interior of the case with the ambient atmosphere which likely contains much dust, dust is likely to pass through the gap between the stem 5 and the window 4a and thus enter the case 4. If the notch is removed in order to avoid such a problem, air cannot move into and out from the cover 7 and so the stem can no longer be easily moved upwardly and downwardly.